With the control of a delivery pressure of a high pressure liquid delivered from an electrically-operated reciprocating plunger pump employing a servo motor or the like as a driving source, it has been the common practice to effect the control by controlling a feed rate of the plungers reciprocating within the cylinder.
As a first method for such pressure control of a high pressure liquid, a method has been known widely in which an actual delivery pressure value detected by a pressure sensor mounted on the plunger pump is fed back so that its deviation from a preset pressure value as the desired value is determined and converted to a speed signal thereby adjusting the rotational speed of the servo motor or the feed rate of reciprocating motion of the plungers by a proportional-plus-integral control (PID control) based on the deviation so as to make the actual delivery pressure value coverage to the desired value.
On the other hand, a second method is such that the control is effected by an ON-OFF control in which while repeating start and stop of the servo motor, the feed rate of reciprocating motion of the plungers is varied so as to feed back and converge the actual delivery pressure value to a preset pressure value.
However, these conventional pressure control methods have the following problems. Where the pressure control is effected by the PID control, due to the nature of the PID control, the control readily and acutely responds to disturbances causing rapid acceleration and deceleration of the reciprocating motion of the plungers. Particularly, there is a problem that since the plunger stroke length of the plunger pump is short, a longer time is required until the actual delivery pressure value reaches a stable state even though acceleration and deceleration are effected frequently. Another problem is that since the control tends to be easily affected by disturbances, even after its stabilization, the actual delivery pressure value tends to vary easily and it is difficult to maintain a constant pressure value.
On the other hand, the pressure control method by the ON-OFF control has a problem that the direction of stroke of the plungers repeatedly in a complicated manner due to frequent start and stop of the servo motor and thus the maintenance of a pressure value at a constant value cannot be expected after its stabilization. There is another problem that the complicated start and stop of the servo motor has the effect of increasing the mechanical burden on the driving system including the belt, pulleys, etc., and reducing the life of the device.
It is to be noted that in a reciprocating pump such as a plunger pump, its delivery pressure of a high pressure liquid will be determined unambiguously by a feed rate of the reciprocating plungers if the nozzle diameter is fixed. Thus, if the feed rate of the plungers can be maintained constant, the delivery pressure value can also be maintained in a stable state.